This is a double-sided circuit board with BGA on one side (B) and resistors on the other side (A).

By Frank Silva, Vice-President of Sales, MatriX-FocalSpot, Inc., San Diego, CAAutomated x-ray inspection (AXI) is an effective means of evaluating solder-joint integrity in support of surface-mount-technology (SMT) process monitoring and defect detection prior to in-circuit or functional testing of printed-circuit boards (PCBs) and other electronic products. Although automated-optical-inspection (AOI) systems and solder-paste-inspection equipment (SPI) are used throughout SMT manufacturing, neither approach is as effective as x-ray inspection at finding defects following the reflow stage of manufacturing. In addition, neither approach is effective at dealing with bottom terminated connections (BTCs) on a PCB. Fortunately, MatriX Technology GmbH (Munich, Germany) has evolved AXI technology not only in terms of image acquisition techniques by software automation, advanced three-dimensional (3D) reconstruction technology, defect detection algorithms, and factory floor traceability solutions. The firm's technology and equipment, is used throughout Tier 1 electronics companies and used for high-reliability, high-value electronics assembly around the world.

Two of the key assembly considerations for SMT designs are imaging of circuit boards with solder joints and bottom terminated components and dealing with double-sided assemblies (DSAs). In the first case, two-dimensional (2D) transmission imaging provides high resolution and fast imaging combined with easing programming and setup. Transmission imaging is limited on some boards, however, such as dense double-sided circuit boards where certain types of defects, such as head-in-pillow (HIP) or pin-in-paste solder joints, must be detected. By working closely with customers and their requirements, MatriX developed algorithms for off-axis transmission imaging in two-and-one-half dimensions (2.5D), identifying specific types of measurements to solve detection issues related to head-in-pillow defect formations and pin-in-paste barrel fill soldering.

Laminography uses geometric focusing by synchronizing the circular motion of an x-ray source.

In the second case, inspecting DSAs with an x-ray system requires special consideration, since solder joints on sides A and B of a circuit assembly can overlap, causing "x-ray shadowing" and resulting in a lack of solder joint x-ray test access. Effective software technology is needed to deal with a DSA board where many areas overlap, so that different components can be separated and treated independently, as if they were on side A without connection to side B or vice versa.

To inspect DSAs, imaging reconstruction techniques had to be developed to separate top-sided from bottom-sided solder joints. These techniques were developed by mathematicians, to deal with medical imaging problems, where multiple shapes and structures overlap and interfere with each other. The term three-dimensional (3D) AXI, while widely used in the industry, is a confusing and misleading term. For AXI use, cross-sectional x-ray imaging would be a more appropriate descriptive term.

Reconstruction TechniquesA horizontal x-ray cross-section can be obtained from two or more images projected from different directions. A number of reconstruction techniques are available to process these images, originating from medical imaging technologies, to separate overlapping "objects" such as solder joints. These techniques include laminography, digital laminography, tomosynthesis, algebraic reconstruction technique (ART), simultaneous ART (SART), and planar CT (pCT), all developed to separate overlapping objects to treat them independently of each other. The imaging separation process employs off-axis image projections and SART enables these objects (such as solder joints) to be "sliced" into horizontal cross-sections at different height locations from the surface of the circuit board. Unfortunately, all imaging reconstruction techniques degrade image quality and contrast compared to a standard transmission image. Some approaches leave fewer artifacts after reconstruction than others.

Enter LaminographyLaminography is a technique used in the 5DX Series of AXI systems from MatriX Technologies. The technology was developed by a former provider of AXI equipment, Agilent Technologies (www.agilent.com), a firm which has since exited the market. The technique uses geometric focusing by synchronizing the circular motion of the x-ray source and detector. Unfortunately, this analog image reconstruction technique produces poor image quality with a high degree of unwanted artifacts. For each slice under study, the board must be repositioned up or down within the slice plane, motions which also create image quality issues. Some AXI systems employ digital tomosynthesis (DT), something of an improvement on laminography. DT relies on a set of off-axis or oblique images stored and synthesized through computational processing. To separate top and bottom components, these angled projections are digitally shifted from each other. The result is improved image quality with fewer shadowing artifacts. Unlike laminography, DT allows multiple two-dimensional (2D) cross-sectional slices to be extracted from the original projections.

Optimum SeparationFor optimum visual separation of top and bottom PB components, MatriX Technologies has implemented two different types of reconstruction techniques into its test equipment. The firm has developed a patent-pending image-processing technique called Slice Filter Technology (SFT) to address DSA setup in a single reflow line configuration. In a single reflow line, AXI can be used to quickly and easily inspect side A of a board in transmission mode. Overlapping solder joints presented by leaded components on side B over the top of Side A create a loss of test access. With SFT, during the process of inspecting side A, a master image or dynamic image of side A can be stored in digital memory. Side B can be tested independently of side A, by subtracting side B from the master or dynamic image of side A or by using 2D imaging. The SFT approach can be used for all views or selectively.

The three-dimensional (3D) simultaneous algebraic reconstruction technique (3D-SART) models the entire physical process of x-ray projections by a system of linear equations. This is an iterative approach, slower than laminography or DT, but suffering fewer artifacts than those other approaches and therefore better solder joint defect detection with fewer false calls. By implementing 3D-SART with a graphical processing unit (GPU), MatriX has achieved fast image reconstruction and enhanced processing speeds; these improvements will continue as GPUs gain in speed. The latest AXI system from MatriX, the X3, employs transmission imaging, off-axis or oblique viewing, slice filter technology, and 3D-SART reconstruction techniques. The flexible system allows operators to deploy the technologies selectively to provide optimum AXI inspection capabilities.

Software AutomationSoftware automation is important for shortening the total program development time from computer-aided-design (CAD) input data and model creation to establishing inspection strategies based on the CAD data and defect detection through advanced algorithmic "solder feature" measurements.

Matrix has engineered two different approaches for program development. The first, Smart Rules, allows quick program development for even NPI environments. The AXI program can be trained with only one circuit board. For higher board volumes, mainly in environments in which more time is available to develop a more robust and deeper defect detection protocol for the highest quality standards, the second approach for program development created by MatriX is called AutoRule. An AXI programmer can implement either one of these strategies during program development.

The latest system from MatriX Technology, the X3, offers a variety of technologies that can be used together or separately for optimum visual inspection.

By applying the Smart Rule approach, the AXI software automatically finds the upper and lower limit "rule thresholds" by isolating the data, taking into account the average and standard deviations. Smart Rule includes a software tool to automatically measure process stability. This tool automatically grades, displays, and highlights which measurements have the lowest variation (are stable) from those with larger variations (are less stable). To make Smart Rules more robust, measurements can be combined into logical statements such that a solder joint would only fail only if measurement A "and" measurement "B" fail. This level of software automation worked to separate the lowest solder measurements (marginal or defective) from the normal histogram data distribution considered normal variation, yet good solder joint.

Systematic ApproachIn the case of an OEM electronics assembler, where longer production runs are more of the norm compared to an EMS environment, program development can take a more systematic approach to achieve the highest defect detection and lowest false calls. By using a technique called the Defect Tree Classificator (DTC), MatriX has developed an effective technique for defect detection. It requires preclassified defect data to be processed, such as:

Missing conditions created by reflowing solder paste without components.

Insufficient or no solder conditions created by taping components onto the circuit board without solder.

Lifted conditions which are created by reflowing solder paste, placing tape over those pads, and then taping parts on top.

Standard production process boards.

The DTC approach allows rules to be automatically generated to separate good from defective solder joints.

After "defective" boards have been run through the inspection system, normally 5 to 10 times, the entire data set can be run through the DTC to automatically separate the defective data from the good data to establish detection guidelines, such as shorts, opens, insufficient, excess, no reflow, missing, off-position, voiding, and solder balls.

Finding DefectsHidden solder joints or bottom terminated connections (BTC) are one of the key decision factors for deploying x-ray inspection equipment. In contrast, AOI systems are suitable for inspecting BGAs, QFNs, package-on-package (POP) devices, pin-in-paste barrel fill, devices under shields, and heal fillets on gull-wing devices. POP devices pose the same issues as DSAs. Inspecting each package separately requires a reconstruction techniques to separate the two packages and then horizontally cross-section them to independently test the top package from the bottom package. The 3D-SART technique produces high-quality images with low artifacts in all of these challenging inspection cases.

Using a 2D transmission x-ray mode, head-in-pillow (HIP) defects cannot be properly detected. Until recently, trying to find HIP defects required horizontal cross-sectioning reconstruction methods. These methods do not adequately ensure HIP detection as other complications arise, namely finding the top surface of the board consistently and the resulting shadows due to reconstruction techniques. But Matrix has developed a robust HIP detection simply using off-axis (2.5D) imaging views and specific polar coordinate algorithms to measure HIP features which were identified to correlate to strong HIP defect detection.

Filling degree is an important parameter to control for pin-in-place (PIP) or thru-hole connectors. Using advanced algorithms which work from off-axis (2.5D) images, the MatriX PIP algorithms can measure the filling level throughout the entire barrel as well as the top side and bottom segments following IPC-6010 specifications (which require a minimum filling level of 75 percent). In short, AXI Technology is evolving to help SMT assembly manufacturers better meet their inspection needs.